The LUNA experiment: direct measurement of thermonuclear cross sections of astrophysical interest

Alessandra Guglielmetti Universita’ degli Studi di Milano and INFN, Milano, ITALY

Laboratory Underground Nuclear Astrophysics

Outline: -Nuclear Fusion reactions in stars -Why going underground

-The Luna Experiment: most important results - On-going measurements and future perspective

p + p d + e+ + ne

d + p 3He + g

3He +3He a + 2p 3He +4He 7Be + g

7Be+e- 7Li + g +ne 7Be + p 8B + g

7Li + p a + a 8B 2a + e++ ne

84.7 % 13.8 %

13.78 % 0.02 %

pp chain

Produces energy for most of the life of the stars

4p 4He + 2e+ + 2ne + 26.73 MeV

Hydrogen burning

Nuclear reactions in stars

Sun: T= 1.5 107 K kT = 1 keV<< EC (0.5-2 MeV)

3He(3He,2p)4He 21 keV

d(p,g)3He 6 keV

14N(p,g)15O 27 keV

Reaction E0

3He(4He,g)7Be 22 keV

Cross section and astrophysical S factor

Gamow energy region

S(E) )/EZ29.31exp(- E

1 (E) 21 Z

Astrophysical factor

Cross section of the order of pb!

Gamow factor EG

S factor can be extrapolated to zero energy but if resonances are present?

Sub-Thr resonance

Extrapol. Mesurements

Narrow resonance

Non resonant process

Tail of a broad resonance

Danger in extrapolations!

Sun Luminosity (irradiated energy per time) = 2 ·1039 MeV/s Q-value (energy for each reaction) = 26.73 MeV

Reaction rate = 1038 s-1

Laboratory Rlab= ··Ip··Nav/A ~ 10 % IP ~ mA ~ g/cm2 pb < < nb

event/month < Rlab < event/day

Underground Laboratory

Rlab > Bcosm+ Benv + Bbeam induced

Environmental radioactivity has to be considered underground shielding

Beam induced bck from impurities in beam & targets high purity

Cross section measurement requirements

1,00E-06

1,00E-05

1,00E-04

1,00E-03

1,00E-02

1,00E-01

1,00E+00

0 2000 4000 6000 8000 10000

Eg[keV]

coun

ts

1,00E-06

1,00E-05

1,00E-04

1,00E-03

1,00E-02

1,00E-01

1,00E+00

0 2000 4000 6000 8000 10000

Eg [keV]

coun

ts

3MeV < Eg < 8MeV: 0.5 Counts/s

3MeV < Eg < 8MeV 0.0002 Counts/s

GOING UNDERGROUND

HpGe

LUNA site

LUNA 1 (1992-2001)

50 kV

LUNA 2 (2000…)

400 kV

Laboratory for Underground

Nuclear Astrophysics

Radiation LNGS/surface

Muons

Neutrons

10-6

10-3

LNGS (shielding 4000 m w.e.)

LUNA MV 2012 ?

400 kV Accelerator : E beam 50 – 400 keV

I max 500 A protons I max 250 A alphas

Energy spread 70 eV Long term stability 5eV/h

Laboratory for Underground Nuclear Astrophysics

CNO Cycle

T>1.6 107 K M>1.1 Solar masses 14N(p,g)15O is the slowest reaction and determines the rate of energy production Its cross section influences: •CNO neutrino flux solar metallicity •Globular cluster age

12C 13N p,g

b-

13C

14N

p,g

15O

b+

15N

p,a

p,g

S 14,1 /5

S 14,1 x5

Standard CF88

Globular cluster age

factor 10 !

Angulo

Schröder

14N(p,g)15O: the bottleneck of the CNO cycle

Transition

(MeV)

Schröder et al.

(Nucl.Phys.A 1987)

Angulo et al.

(Nucl.Phys.A 2001)

RC / 0 1.55 ± 0.34 0.08 ± 0.06

S(0) [kev-b] 3.20 ± 0.54 1.77 ± 0.20

factor 20 !

High resolution measurement (2004)

Solid target + HPGe detector

single γ transitions Energy range 119-367 keV

summing had to be considered

Gas target+ BGO detector

high efficiency total cross section

Energy range 70-230 keV

High efficiency measurement (2006)

CNO neutrino flux decreases of a factor 2 Globular Cluster age increases of 0.7 – 1 Gyr

S0(LUNA) = 1.61 ± 0.08 keV b

3He(4He,g )7Be

John Bahcall e M. H. Pinsonneault, astro-ph/0402114v1, 2004:

The rate of the reaction 3He(4He,g)7Be is the largest nuclear physics contributor to the uncertainties in the solar model predictions of the neutrino fluxes in the p-p chain. In the past 15 years, no one has remeasured this rate; it should be the highest priority for nuclear astrophysicists.“

F(8B) ~ (1+dS11) -2.73 (1+dS33) -0.43 (1+dS34) 0.85 (1+dS17) 1.0

(1+dSe7) -1.0 (1+dS1,14) -0.02

where fractional uncertainty dS11 DS11/S11(0)

Eg = 478 keV

3He(a,g)7Be(e,n) 7Li*(g)7Li

Eg =1586 keV + Ecm (DC 0);

Eg = 1157 keV + Ecm (DC 429)

Eg = 429 keV

0.3

0.4

0.5

0.6

0.7

0.8

S(0

) [k

eV

b]

Parker

1963

Robertson

1983

Hilgemeier

1988

Alexander

1984

Osborne

1982

Kräwinkel

1982

Nagatani

1969

Volk

1983

Osborne

1982

Hilgemeier

1988

Nara Singh

2004Average

g prompt

Average activa.

Prompt g Activation

The two techniques show a 9% discrepancy

3He recirculating gas target p=0.7mbar

Si-monitor for target density measurement (beam heating effect)

Collimated HPGe detector to collect g ray at 55

0.3 m3 Pb-Cu shield suppression five orders of magnitude below 2MeV

Removable calorimeter cap for offline 7Be counting

Luna measurement: both techniques and accuracy of 4-5%

Results

Uncertainty due to S34 on neutrinos flux: Φ(8B) 7.5% 4.3% Φ( 7Be) 8% 4.5%

B.N. Singh2004

LUNA data Prompt activation

in Solar fusion cross sections II: arXiv:1004.2318v3 based on LUNA and successive measurements:

S34= 0.56

0.02 (exp)

0.02 (model) keV b

S34 (LUNA) =0.567±0.018±0.004 keV b

reaction Q-value (MeV)

Gamow energy (keV)

Lowest meas. Energy (keV)

LUNA limit

15N(p,g)16O 12.13 10-300 130 50

17O(p,g)18F 5.6 35-260 300 65

18O(p,g)19F 8.0 50-200 143 89

23Na(p,g)24Mg 11.7 100-200 240 138

22Ne(p,g)23Na 8.8 50-300 250 68

D(a,g)6Li 1.47 50-300 700(direct)

50(indirect)

50

CNO cycle

Ne-Na cycle

BBN

In progress

In progress

completed!

LUNA present program

to be completed presumably by 2014

He 3 He

4

Be 7

Li 7

H D p

n

2

1

3 4

2

8

9

6

7

11

12

10

3 5

Li 6

13

1. n p + e- + n

2. p + n D + g

3. D + p 3He + g

4. D + D 3He + n

5. D + D 3H + p

6. 3H + D 4He + n

7. 3H + 4H 7Li + g

8. 3He + n 3H + p

9. 3He + D 4He + p

10. 3He + 4He 7Be + g

11. 7Li + p 4He + 4He

12. 7Be + n 7Li + p

13. 4He + D 6Li + g

BBN: production of the lightest elements (D, 3He, 4He, 7Li, 6Li) in the first minutes after the Big Bang

Apart from 4He, uncertainties are dominated by systematic errors in the nuclear cross sections

The 6Li case

Constant amount in stars of different metallicity (age) 2-3 orders of magnitude higher than predicted with the BBN network (NACRE) The primordial abundance is determined by: 2H(a,g)6Li producing almost all the 6Li 6Li(p,a)3He destroying 6Li well known

6Li

7Li BBN prediction

BBN prediction

[F. Hammache et al., Phys. Rev. C 82, 065803 (2010)]

Direct measurements: Robertson et al. E > 1 MeV Mohr et al. around the 0.7 MeV resonance Indirect measurements: •Hammache et al. upper limits with high energy Coulomb break-up At LUNA direct measurements at the energies of astrophysical interest

LUNA

BBN energy region 0 0.5 1.0 MeV

Available data

The beam-induced background

d

n

3He

α

α

d

d

- neutron background generated by d(a,a)d Rutherford scattering followed by d(d,n)3He reactions

-> (n,n’γ) reactions on surrounding materials (Pb, Ge, Cu)

-> much higher γ-ray background in the RoI for the d(α,γ)6Li DC transition (~1.6 MeV)

Reduced gas volume: pipe to minimize the path of scattered 2H and hence to minimize the d(d,n)3He reaction yield - HPGe detector in close geometry: larger detection efficiency and improved sygnal-to-noise ratio - Silicon detectors to measure 2H(2H,p)3H

Experimental set-up

Germanium Detector

Silicon Detectors to detect D(D,p)3H protons

Steel pipe to

minimize D+D

reactions yield

LUNA measurement (preliminary)

still running to double the acquired statistics

230 h at 400 keV, 285 h at 280 keV

LUNA MV Project April 2007: a Letter of Intent (LoI) was presented to the LNGS Scientific Committee (SC) containing key reactions of the He burning and neutron sources for the s-process: 12C(a,g)16O 13C(a,n)16O 22Ne(a,n)25Mg (a,g) reactions on 14,15N and 18O These reactions are relevant at higher temperatures (larger energies) than reactions belonging to the hydrogen-burning studied so far at LUNA

Higher energy machine 3.5 MV single ended positive ion accelerator

Possible location at the "B node" of a 3.5 MV single-ended positive ion accelerator

• In a very low background

environment such as LNGS, it is

mandatory not to increase the

neutron flux above its average value

Study of the LUNA-MV neutron shielding by

Monte Carlo simulations

OnC 1613 ),(a

α beam intensity: 200 µA Target: 13C, 2 1017at/cm2 (99%

13C enriched) Beam energy(lab) ≤ 0.8 MeV

MgnNe 2522 ),(aα beam intensity: 200 µA Target: 22Ne, 1 1018at/cm2 Beam energy(lab) ≤ 1.0 MeV

OC 1612 ),( gaOnC 1613 ),(a from α beam intensity: 200 µA

Target: 13C, 1 1018at/cm2 (13C/12C = 10-5) Beam energy(lab) ≤ 3.5 MeV

• Maximum neutron production rate : 2000 n/s

• Maximum neutron energy (lab) : 5.6 MeV

Geant4 simulations for neutron fluxes just outside the experimental hall and on the internal rock walls

First results

Round Table "LUNA-MV at LNGS" 10th-11th February 2011

http://luna.lngs.infn.it/luna-mv

35 scientists from Europe, USA and Asia: •Status of similar projects in Europe and USA •Description of the LUNA MV project (site, machine, shielding,...) •Astrophysical importance of the envisaged reactions •Experimental open problems •Discussion

Two documents: A) Proceedings B) Brief description of the project and list of "Working packages" to be distributed for adhesions (Aliotta, Fraile, Fulop , Guglielmetti)

Next-generation underground laboratory for Nuclear Astrophysics Executive summary

This document originates from discussions held at the LUNA MV Roundtable Meeting that took place at Gran Sasso on 10-11 February 2011. It

serves as a call to the European Nuclear Astrophysics community for a wider

collaboration in support of the next-generation underground laboratory. To state your interest

to contribute to any of the Work Packages, please add your name, contact details, and WP

number under International Collaboration.

WP1: Accelerator + ion source WP2: Gamma detectors WP3: Neutron detectors WP5: Solid targets WP6: Gas target WP7: Simulations WP8: Stellar model calculations

THE LUNA COLLABORATION Laboratori Nazionali del Gran Sasso

A.Formicola, M.Junker Helmoltz-Zentrum Dresden-Rossendorf, Germany

M. Anders, D. Bemmerer, Z.Elekes INFN, Padova, Italy

C. Broggini, A. Caciolli, R.Menegazzo, C. Rossi Alvarez INFN, Roma 1, Italy

C. Gustavino Institute of Nuclear Research (ATOMKI), Debrecen, Hungary

Zs.Fülöp, Gy. Gyurky, E.Somorjai, T. Szucs Osservatorio Astronomico di Collurania, Teramo, and INFN, Napoli, Italy

O. Straniero Ruhr-Universität Bochum, Bochum, Germany

C.Rolfs, F.Strieder, H.P.Trautvetter Seconda Università di Napoli, Caserta, and INFN, Napoli, Italy

F.Terrasi Università di Genova and INFN, Genova, Italy

P.Corvisiero, P.Prati Università di Milano and INFN, Milano, Italy

M.Campeggio, A.Guglielmetti, D. Trezzi Università di Napoli ''Federico II'', and INFN, Napoli, Italy

A.di Leva, G.Imbriani, V.Roca Università di Torino and INFN, Torino, Italy

G.Gervino University of Edinburgh M. Aliotta and D. Scott